Is matching output tubes necessary if I can adjust bias for each of four tubes ?
@inna Yes! Its one thing to set the bias point, which defines the operating point of the tube. But the tube has a dynamic operating curve; ideally each of the tubes have matching curves to minimize distortion at all power levels; IOW they are working perfectly in tandem all the time. This is true even if the power tubes are biased class A1.
To do this properly, the tubes are matched on something called a curve tracer. They can be created using a computer platform, like this one, which is a kit that allows you to create a very accurate test platform. There are a number of kits and construction articles on how to build one for yourself. One that is already built is likely going to be a couple of grand$.